1. Field of the Invention
This invention relates to circuit breakers which blow open under short-circuit conditions. More particularly, the invention is directed to such a circuit breaker in which the blow open latch forms part of the power circuit between the pivoted contact arm and the trip mechanism.
2. Background Information
A common type of molded case circuit breaker has for each pole a fixed contact attached to a fixed conductor within the circuit breaker, and a moveable contact carried by a pivotally mounted contact arm. The contact arm is rotated by an operating mechanism between a closed position in which the moveable contact engages the fixed contact and an open position in which the contacts are separated. The operating mechanism can be actuated manually through a handle or automatically through a trip mechanism. Typically, the trip mechanism incorporates a thermal or delayed trip function which opens the contacts in response to a persistent overcurrent condition, and a magnetic or instantaneous trip function which opens the contacts in response to a high instantaneous current. In some trip mechanisms the thermal trip function is performed by a bimetal and the magnetic trip function is performed by an armature which is attracted by the magnetic field generated by the high current. In other types of trip mechanisms, the delayed and instantaneous trip functions are performed electronically, typically by a microprocessor.
Some of these molded case circuit breakers incorporate a blow open feature which allows the contact arm to open rapidly in response to very high currents, such as short circuit currents, without waiting for the trip mechanism to respond. Typically, this is accomplished by aligning a portion of the fixed conductor parallel to the contact arm, with the contacts closed, so that the current flows in oppositely directed adjacent paths in the fixed conductor and the contact arm. The resultant magnetic fields generate repulsion forces which, for the high currents produced by a bolted fault, are sufficient to rapidly rotate the contact arm open.
The contact arm is mounted in a support frame for rotation by the operating mechanism. The contact is pivotally connected to the support frame so that in response to very high short circuit currents, the contact arm is blown open or is rotated relative to the support frame by the above-mentioned repulsion forces. The magnitude of the current at which the contact arm is blown open is set by a blow open latch in the form of an electrically non-conductive latch member spring biased against a blow open camming surface on the pivoted end of the contact arm. This spring also permits overtravel of the operating mechanism to provide contact pressure for the contacts and to accommodate for contact wear.
The current in the protected circuit flows in the circuit breaker from a line terminal through the fixed conductor, the fixed contact, the moveable contact and the contact arm from which it passes to the trip mechanism and then to a load terminal. Typically, the circuit between the pivoted contact arm and the trip mechanism is completed by a flexible shunt which is usually a braided copper conductor. Often, this flexible shunt is connected to the contact arm between the moveable contact and the pivot, passes around the support frame and is then connected to the free end of the bimetal in the trip mechanism. This flexible shunt takes up room within the circuit breaker molded casing, and having a relatively high resistance, produces heat within the circuit breaker.
There is a need therefore, for a circuit breaker with an improved blow open capability.
There is a related need for such an improved circuit breaker having a connection between the pivoted contact arm and trip mechanism which does not produce as much heat as current circuit breakers of this type.
There is another need for such an improved circuit breaker which does not require as much space within the circuit breaker housing so that the circuit breaker may be made smaller for the same current rating or accommodate a higher current rating in the same space.